Among electrochemical energy devices, examples of cells having a 1.5 V or higher charging or discharging voltages include lithium primary cells, secondary cells, lithium ion secondary cells, and electro-double layered capacitors of large-volumetric types. Water cannot be used as an electrolyte solvent for these high voltage electrochemical energy devices because water is electrolyzed at such high voltages. Therefore, an aprotic solvent, such as alkyl ester carbonate, alkyl ether in which carrier electrolyte is dissolved, is used for the electrolyte solvent. Similarly, a non-aqueous electrolytic solution is used even for a cell which has a voltage of no greater than 1.5 V, because when electrodes in which occluded or discharged lithium is used, the lithium species active in electrodes react easily with water.
However, due to the combustible nature of aprotic solvents there is a risk that upon leaking outside of the cell the electrolyte solution will catch fire because of heat generation caused by unusual charge or discharge. Electrochemical energy devices are commonly used as main electric sources for portable small electronic devices such as notebook computers and mobile phones, or as memory back-up sources for these devices, and they are widely used by ordinary consumers. Accordingly, the tendency of such devices to catch fire is an acute issue. When large-sized electrochemical devices are used as main or auxiliary electric sources for motor-driven automobiles, or as electric power storing stationary devices, the danger of catching fire at emergency is increased.
Conventional methods for making non-aqueous electrolytic solutions to be flame resistant are, for example, as follows:
Japan Unexamined Patent Publication No. 9-293533 discloses a method in which 0.5 to 30 weight percent of fluorinated alkane having 5 to 8 carbon atoms is incorporated into a non-aqueous electrolytic solution. Generally, fluorinated alkanes, particularly perfluorinated alkanes are not combustible and they may impart flame resistance as a result of a blanket of the volatile gas tending to choke off combustion sources. However, the fluorinated alkane has no other beneficial effect to the electrochemical cell other than imparting flame resistance to the solution. Furthermore, fluorinated alkanes, particularly the perfluorinated alkanes, will not readily dissolve in the aprotic solvent as is necessary to create effective electrolytic solutions for use in electrochemical energy devices. Because the incombustible fluorinated alkane phase separates from the combustible aprotic solvent phase, the entirety of the electrolytic solution cannot be said to be flame resistant. In addition, the separated fluorinated alkane phase is likely to be positioned lower than the aprotic solvent phase because of its relatively higher specific gravity. As a result, the incombustible layer phase will not effectively choke a fire in the aprotic phase. Furthermore, because the carrier electrolyte will not dissolve well in the fluorinated alkane phase, ions and electrons are not effectively exchanged and are occluded at the inter-phase region between electrodes and the electrolytic solution, resulting poorly performing electrochemical energy devices.
Japan Unexamined Patent Publication 11-307123 discloses a method of using a hydrofluoroether, such as methyl nonafluorobuthyl ether. The hydrofluoroether itself is incombustible and is readily soluble in a hydrocarbon solvent. Thus, the hydrofluoroether can be used to produce a uniform, non-aqueous electrolytic solution having flame resistant characeristics. However, the flame resistant mechanism derives mainly from the fire choking effect of the volatile constituent of the hydrofluoroether, just as the case with the fluorinated alkane, and the flame resistance is still insufficient. Further, the non-aqueous electrolytic solution should contain a sufficient proportion of hydrofluoroether such as methyl nonafluorobuthyl ether, so that the solution itself may be flame resistant. In fact, the reference teaches that the noncombustible electrolytic solution is obtained by containing 65 volume % or more of methyl nonafluorobuthyl ether based on a total amount of the solvent composition excluding salt. However, the hydrofluoroether tends to be a poor salvation agent of the salt, and as are result such solutions tend to provide less than desired ion conduction properties. Furthermore, when a hydrofluorether-containing non-aqueous electrolytic solution leaks out from a cell or capacitor, the proportion of the hydrofluoroether in the leaked electrolytic solution tends to decrease, in time to the a level where the solution is no longer flame resistant because the hydrofluoroether has a relatively high vapor pressure and a low boiling point, causing it to vaporize quickly. Furthermore, desired flame resistance may tend to be lost as the fire choking blanket of hydrofluoroether gas dissipates, particularly when exposed to high temperature conditions.
Thus, the need exists for a non-aqueous mixture solvent suitable for use in non-aqueous electrolytic solutions that exhibit effective properties of flame resistance, non-combustibility and self extinction of fire, and which do not impair the performance of electrochemical energy devices, and to provide a non-aqueous electrolytic solution containing the solvent.